At the present time, it is common to permit oil wells to flow under their own natural pressure as long as they will do so and then to apply a mechanical reciprocating pump to complete the removal of the oil. This method, although in general use, is cumbersome and unsatisfactory. Because suction will only raise oil for a distance of some 35 feet, it is necessary to have the pump near the bottom of the well so that it can exert pressure instead of suction on the oil coming out of the well. This involves the use of pump rods of lengths of 5,000 feet or greater in many instances and when the pump plunger or the valves become worn, it is necessary to remove the pump from that depth to replace the worn parts. Furthermore, the collars on the pump rod wear rapidly and all the pump parts do likewise because of the small particles of grit that remain in the oil and the whole device is mechanically inefficient because of the relatively long pump rods that must be reciprocated to perform the pumping operation.
When the natural flow of crude oil from a well has ceased or become too slow for economical production, artificial production methods are employed, and in many cases, it is advantageous, at least during the first part of the artificial production period, to employ gas lift. Numerous types of equipment for producing oil by gas lift are available, but they all rely upon the same general principles of operation. In the usual case, dry gas consisting essentially of methane and ethane is forced down the annulus between the tubing and the casing and into the oil in the tubing. As the oil in the tubing becomes mixed with gas, the density of the oil decreases, and eventually the weight of the column of the gasified oil in the tubing becomes less than the pressure exerted on the body of oil in the well, and flow of oil occurs at the surface. While in some cases the dry gas may be introduced through the tubing so as to cause production through the annulus, this is not preferred unless special conditions are present.
One known gas lift technique is employed in oil wells, which have difficulty in producing naturally, that is, wells in which the formation pressure is not sufficient to cause the well to produce at an acceptable volume. This gas lift technique injects gas into the casing, which has been sealed or packed off at the bottom of the hole relative to the production tubing. A gas lift valve is placed in the production tubing at the production level, and the gas lift valve permits the gas to be injected into or bubble very slowly into the fluid being produced from the well. This gas then makes the fluid in the production tube somewhat lighter and, hence, the natural formation pressure will be sufficient to push the fluid up and out of the well. This means that the well can be produced at a greater rate. This gas lift technique is known as continuous gas lift.
A further adaptation of this gas lift technique is known as intermittent gas lift. In this technique, rather than letting the gas enter the production tube slowly, the gas is injected into the production tubing very quickly, in short bursts, thereby forming a large slug of fluid in the production tubing above the injected gas bubble. The gas bubble then drives the slug of fluid in the production tubing upwardly. The technique is repeated successively, thereby producing successive slugs of fluid at the wellhead.
Another type of gas lift tool involves a procedure where a string of production fluid extending from the surface to the zone of interest is provided with a number of gas lift valves positioned at spaced intervals along the length of the tubing. Gas is injected from the annulus between the tubing and well pipe through the gas lift valves and into the tubing for the purpose of forcing liquid upwardly to the surface and ultimately into a flowline that is connected with the production tubing. Gas lift systems for liquid production are quite expensive due to the cumulative expense of the number of gas lift valves that are ordinarily necessary for each well. Moreover, each of the gas lift valves must be preset for operation at differing pressures because of the vertical spacing thereof within the tubing string and because the valves must function in an interrelated manner to achieve lifting of liquid within the tubing string.
A common problem encountered in producing a well by gas lift arises in starting the flow of oil. The gas lift systems available on the market generally comprise of an arrangement of valves in the tubing whereby the gas may be introduced first into the upper part of the column of oil in the tubing and then at lower points so that it is not necessary to raise the entire column of oil in the tubing a substantial distance prior to the mixture of the gas with the oil. These systems of gas lift satisfactorily accomplish the object of lessening the problem of starting production. When the crude oil being produced tends to deposit paraffin, the presence of the valves makes the job of cleaning paraffin from the tubing more difficult.
In the patent literature, there are a variety of proposals for gas lift apparatus and the like, and the following U.S. patents disclose various apparatus for producing crude oil using gas:
______________________________________ U.S. Pat. No. Patent Date of Issue ______________________________________ 1,547,197 Arbon 28 July, 1925 2,034,798 Clark 24 March, 1936 2,275,947 Courtney 10 March, 1942 2,293,196 Crump 18 August, 1942 2,380,639 Eris 31 July, 1945 2,463,317 Sanders 1 March, 1949 3,234,890 Adams et al 15 Feb., 1966 3,718,407 Newbrough 27 Feb., 1973 3,814,545 Waters 4 June, 1974 3,873,238 Elfarr 25 March, 1975 4,267,885 Sanderford 19 May, 1981 4,390,061 Short 28 June, 1983 4,738,313 McKee 19 April, 1988 ______________________________________
The Arbon patent, U.S. Pat. No. 1,547,197 provides a flow of gas through the annulus between the inner and outer tubing, to gasify the production liquid. Gasification jets inject the gas into the oil flow, and are provided with check valves to prevent back glow of production liquid. However, the production oil flows through a single duct, with the gas injected at the sides.
The Courtney patent, U.S. Pat. No. 2,275,947 has different size check valves for different lift off pressures, allowing the flow through the gasification orifices to be varied depending on the pressure of the supplied gas. A standing check valve located at the bottom of the mechanism prevents downward flow of production oil.